Carbonate reservoirs are difficult to exploit because of their heterogeneous nature. A major challenge in carbonate environments is therefore to map these heterogeneities which have a strong impact on oil and gas production.
In many carbonate reservoirs, the porosity of the rock (i.e. matrix porosity) is high enough to contain large amounts of oil in place, but the permeability is mainly provided by fracture corridors, not by the intrinsic nature of the rock matrix. In other reservoirs, the oil in place is found primarily in caves and conduits formed in the rock formation by infiltration and action of rain water (so-called karst formations).
Therefore, the ability to detect these heterogeneities and possibly characterize their properties, i.e. obtaining three dimensional maps of their geometry and characteristics, is essential in these environments.
To obtain images of the subsurface, the seismic method is often used, which consists in creating and sending seismic waves in the ground using sources such as explosives or vibrator trucks on land, or airguns offshore. The seismic waves penetrate the ground and get bounced, or reflected off major geological discontinuities in the subsurface. As a result, they come back to the surface, where they are recorded using arrays of three component geophones (on land), or hydrophones (offshore) which are regularly distributed to cover areas of several square kilometers.
Seismic reflections assume that local planes are large compared to the seismic wavefront. When the subsurface contains edges and short-scale heterogeneities, the wavefront undergoes diffractions rather than reflections.
Diffraction effects are typically present with carbonate reservoirs, because of the characteristics mentioned above, i.e. the presence of faults, fissures, conduits, caves etc.
The importance of diffracted waves for obtaining better images of subsurface carbonate-type reservoirs has long been recognized.
Typically, diffracted energy is one or even two orders of magnitude weaker than the reflected one and it is not easy to distinguish diffracted events in a seismic dataset or a diffraction image in a seismic image. Therefore, diffracted and reflected energy have to be separated properly.
A suitable domain for performing this separation seems to be the post-migration dip angle domain as disclosed by Landa et al. “Separation, imaging, and velocity analysis of seismic diffractions using migrated dip angle gathers”, SEG Expanded Abstracts, vol. 27, pages 2176-2180, 2008. In this document, reflection and diffraction events are separated in the dip angle domain using a plane-wave-destruction method, described by Fomel: “Applications of plane-wave destruction filters”, Geophysics, 67, 1946-1960, 2002, requiring accurate estimation of the velocity model used for the migration.